Carbon fibers have received increasing attention as reinforcing materials due to their light weight and excellent mechanical properties such as strength and modulus of elasticity as compared with conventional reinforcing materials such as glass fibers. They are widely used at present in the production of shafts for golf clubs, fishing rods and racket frames, for example. It has been desired to reduce their production costs and also to develop high performance carbon fibers so that they can be used as industrial materials in the fields of air crafts and space technology, for example.
It is known that impurities such as metals are responsible for the formation of defects in carbon fibers, i.e., a reduction in their strength and thermal stability. Therefore, in order to produce high performance carbon fibers, various investigations have been made to produce acrylonitrile-based precursors which are free of such impurities.
In the production of carbon fibers, various techniques are employed to proceed uniformly and rapidly the formation of naphthyridine-like structures in the carbon fibers at an initial stage of the production thereof, i.e., a so-called treatment to impart flame resistance. Of these techniques, a method is widely used in which carboxyl group-containing polymerizable unsaturated monomers are copolymerized with acrylonitrile. Carboxyl groups contained in the polymers, however, tend to catch metals, in particular, alkali metals at the steps of polymerization, spinning, and drawing, thereby causing the abovedescribed defects and in turn a reduction of strength of the carbon fibers. In order to overcome this problem, Japanese Patent Publication No. 33211/76 discloses a method in which some of the carboxyl groups are substituted with ammonium ions. This method, however, has several disadvantages. For example, a degree of substitution of the terminal hydrogen of the carboxyl group is from 0.1 to 15%, and since the substitution process is carried out as a post-treatment after the spinning process, it not only results in increasing the number of steps but also exerts adverse influences on the ultimate carbon fibers.